Thermal imaging cameras are used in a variety of situations. For example, thermal imaging cameras are often used during maintenance inspections to thermally inspect equipment. Example equipment may include rotating machinery, electrical panels, or rows of circuit breakers, among other types of equipment. Thermal inspections can detect equipment hot spots such as overheating machinery or electrical components, helping to ensure timely repair or replacement of the overheating equipment before a more significant problem develops.
Thermal imaging cameras generally include an infrared (IR) camera module that incorporates an array of infrared sensors or detectors for sensing IR radiation and generating corresponding electrical signals. One well-known type of infrared detector is the “bolometer,” which operates on the principle that the electrical resistance of the bolometer material changes with respect to the bolometer temperature, which in turn changes in response to the quantity of absorbed incident IR radiation. These characteristics can be exploited to measure incident infrared radiation on the bolometer by sensing the resulting change in its resistance.
Microbolometer detector arrays may be used to sense a focal plane of incident IR radiation. Each microbolometer detector of an array may absorb any radiation incident thereon, resulting in a corresponding change in its temperature, which results in a corresponding change in its resistance. With each microbolometer functioning as a pixel, a two-dimensional image or picture representation of the incident IR radiation may be generated by translating the changes in resistance of each microbolometer into a time-multiplexed electrical signal that can be displayed on a monitor or stored in a computer. As used herein, the term “pixel” is equivalent to the terms “detector,” “sensor,” and more specifically in some cases, “microbolometer.” The circuitry used to perform this translation is commonly known as a Read Out Integrated Circuit (ROIC), and is commonly fabricated as an integrated circuit on a silicon substrate. The microbolometer array may then be fabricated on top of the ROIC. The combination of the ROIC and microbolometer array is commonly known as a microbolometer infrared Focal Plane Array (FPA). Microbolometer focal plane arrays may contain different numbers of detectors. One common example is an FPA with as many as 640×480 detectors.
Methods for implementing an ROIC for microbolometer arrays have used an architecture wherein the resistance of each microbolometer is sensed by applying a uniform electric signal source, e.g., voltage or current sources, and a resistive load to the microbolometer element. The current resulting from the applied voltage is integrated over time by an amplification stage to produce an output voltage level proportional to the value of the integrated current. The output voltage can then converted to a digital signal using an analog-to-digital converter (ADC) and multiplexed with other conditioned bolometer readings to generate an image of the target scene.
One measure of a thermal imaging camera's performance is the camera's dynamic range, which refers to the range of temperatures in a target scene that can be imaged by the camera at any one time. As is known, thermal imaging cameras employing microbolometer FPAs suffer from a necessary compromise in performance between the camera's imaging dynamic range and system noise. In general, if a scene with very large temperature differences is expected to be imaged with the FPA, the overall gain can be set lower so that a wide range of detector responses (corresponding to the wide range of temperatures) can be processed without exceeding circuit maximum and/or minimum signal levels, such as in the amplification stage or the input of the A/D converter. Gains that are too low, though, can decrease the system's signal-to-noise ratio. In addition, low gains may unnecessarily “compress” the histogram of detector responses and thereby decrease the effective resolution of the ADC, as well as leave a portion of the input range thereof unused. On the contrary, while high gains may provide high signal-to-noise performance, gains that are too high can “widen” or “spread” the histogram of detector responses so that output signal values for some microbolometer pixels are out of the input range of the A/D (i.e., greater than Vmax or less than Vmin), thus decreasing the dynamic range.
One method currently used to address this tradeoff is to configure a thermal imaging camera with two or more ranges with corresponding gain settings. The camera can then switch between the two or more ranges/settings depending upon the instantaneous distribution of temperatures in a target scene in an attempt to obtain a best fit between one of the camera ranges and the particular range of temperatures currently being imaged. While this approach has been useful, it often requires frequent shutter firing as the camera circuitry switches between gain settings, which can cause noticeable and undesirable pauses in a thermal imaging display as viewed by a user. In the event that the imager also measures temperature, such an approach also requires calibration of multiple ranges, which can extend the calibration time and build time of a camera in rough proportion to the number of ranges included.